The present invention relates to a bicycle internal hub transmissions, and more particularly to a bicycle internal hub transmission that can be mounted to the frame of the bicycle and that transmits the power from an input member to an output member at a specific selected gear ratio.
Bicycles, particularly recreational bicycles referred to as city cruisers, are inexpensive and are easy to ride, and are thus widely used to commute to work or school or for shopping. With this type of recreational bicycle, an internal hub transmission is sometimes mounted at the rear wheel in order to ride at high speeds over flat terrain or to ride uphill with minimal exertion.
An internal hub transmission generally comprises a hub axle that is fixed to the bicycle frame, a hub shell, a driver, a planet gear mechanism, an operation mechanism and an actuating mechanism. The hub shell rotates around the hub axle and has spoke holes around its outer periphery. The driver is rotatably supported on the hub axle and is linked to the hub cog. The planet gear mechanism is disposed in the housing space of the hub shell and comprises a sun gear formed on the hub axle, a plurality of planet gears that mesh with the sun gear, a ring gear that meshes with the planet gears, and a gear frame that rotatably links the planet gears and is able to rotate around the hub axle. In the case of a three-speed bicycle, this planet gear mechanism has three power transmission paths: a direct drive position that does not go through the planet gear mechanism, an upshift path in which the ring gear is rotated via the planet gears of the planet gear mechanism, and a downshift path in which the planet gears are rotated via the ring gear of the planet gear mechanism. The operation mechanism has an operation rod disposed inside the hub axle such that it can move in the axial direction and a clutch member that moves in conjunction with the operation rod. The clutch member is used for selecting one of the plurality of power transmission paths of the planet gear mechanism.
There are two types of actuating mechanisms for moving the operation mechanism in the axial direction: a bell crank type and direct-pull type that features a chain or cable. A bell crank type actuating mechanism has a support member that is mounted at the end of the hub axle and a swing link that is swingably supported by the support member. One end of the swing link is connected to a shift cable that is linked at its distal end to a shift lever. The other end of the swing link contacts the operation rod protruding from the end of the hub axle, and a shift is performed by pushing the operation rod with the swing link. A direct-pull type actuating mechanism has a chain or cable linked to the distal end of the operation rod. With a direct-pull type actuating mechanism, the chain or cable protrudes from the hub axle end and then curves around and is linked to the shift cable, and a shift is performed by pulling the operation rod. In either case, when the shift lever is operated and the shift cable pulled or played out, the operation rod is moved and the power transmission path is switched by the clutch member.
A direct-pull type actuating mechanism has a simpler construction than a bell crank type, and therefore contributes to lower cost. However, a direct-pull type actuating mechanism involves directly pulling the operation rod with a chain or cable, so the operating efficiency is lower than with a bell crank type, wherein the operation rod is moved by the swinging of a swing link. "Operating efficiency" here refers to the ratio of the operating force actually applied by the rider to the operating force applied to the operation rod. Also, since a direct-pull type actuating mechanism involves linking the operation rod to a chain or cable inside the hub axle, the assembly work, operation rod replacement work, and so on are more difficult than with a bell crank type actuating mechanism. Accordingly, a bell crank type actuating mechanism is used more often for internal hub transmissions, and particularly for three-speed internal hub transmissions.
Regardless of which type of actuating mechanism is used, since the actuating mechanism protrudes from the hub axle end, the actuating mechanism is susceptible to damage if the bicycle falls over. Such damage to the actuating mechanism can in some cases preclude shifting. There is also the danger that the protruding actuating mechanism will hit or snag on objects while the bicycle is being ridden.